7526
J . Phys. Chem. 1989, 93, 7526-7528
examined here may be quite similar. Evidence to that effect is our observation that at a temperature 0.85 f 0.45’ below the melting point, the degree of disorder as measured by the number of gauche bonds per chain is nearly the same for CI7,C25, c36, C50,and c 6 0 (see Table I). This suggests that in this temperature region the disorder in the chain ends is quantitatively similar for the different crystalline n-alkanes.
number of gauche bonds into a highly ordered lattice will disrupt the lattice disproportionally, so that the number of “disordered” methylenes [m*] will be significantly larger than the number of gauche bonds [m(g)]. The ratio of the number of “disordered” methylenes to the number of gauche bonds (minus the background contribution), that is m*/[m(g) - 0.11, evaluated at a temperature just above T,, is in the range 3.0-4.5 for C,7, C25, and C36. This ratio varies for C50 and C60because m(g) changes with temperature. However, near the high-temperature end of temperature region 11, the ratio is around 4.5 for these chains. Temperature Region I l l . This narrow temperature region, in which actual melting occurs, was discussed in the Results section. In summary, it appears that the melting behavior of the n-alkanes
Acknowledgment. We gratefully thank the National Science Foundation (DMR 87-01586) and the National Institutes of Health (GM 27690) for supporting this work. We also thank Professor Leo Mandelkern for valuable discussions and encouragement during the course of this project.
0,- Formatlon In Nafion by Electron Transfer from Ti3+. The Effect of AI3+ as Cocation M. G.Alonso-Amigot and Shulamith Schlick* Department of Chemistry. University of Detroit, Detroit, Michigan 48221 (Received: April 12, 1989)
The superoxide radical anion 0; has been detected by electron spin resonance (ESR) in dry Nafion perfluorinated membranes neutralized by Ti3+. The most likely mechanism for 02-formation is electron transfer from Ti3+. The g values for 02-at 77 K are 2.0029,2.0090, and 2.0193, *0.0005, and are typical of the superoxide radical associated with a Ti4+center. The 0, radical is motionally rigid and stable below 300 K. Above 3 10 K the signal disappears rapidly. Evacuation of the sample results in the appearance of the ESR spectrum of freely rotating molecular oxygen, trapped in the membranes. The rates of 02-formation and of Ti3+disappearance can be followed by ESR and are significantly enhanced by the presence of AI3+ cations, especially for A13+mole fractions larger than 0.5 in the cation mixture. We suggest that this effect is due to the reduced clustering of Ti3+in Nafion, in the presence of A13+. The results presented are thought important for an understanding of isotactic polymerization processes by Ziegler-Natta catalysts.
Introduction The superoxide radical 02-is a ubiquitous and important reactive intermediate in many catalytic processes on metal oxide surfaces.] In addition 02-is also involved in a variety of biological systems; the discovery of an enzyme that catalyzes superoxide dismutation has greatly encouraged the study of the radical and its biological activity.2 In many chemical and biological systems 02-is formed by electron transfer from low oxidation state metal cations such as Cu+, Fe2+, and MoS+. 02- adsorbed by titanium(1V) oxide supported on porous Vycor glass3v4and on metal cations such as vanadium(1V) and palladium(I1) on TiOz (rutile) carriers has also been identified.5s6 Very stable 0 2 - radicals, with a lifetime of at least 2 years, have been obtained in a titania gel matrix by reacting Ti3+(as Tiel3) or titanium metal with H 2 0 2in a Haber-Weiss type reaction.’ Electron spin resonance (ESR) is the method of choice for identifying and characterizing 02-, because of its specificity and sensitivity to species containing unpaired electron spins. Because I6O has zero nuclear spin, the ESR spectrum of 0; is very simple and exhibits only g anisotropy. The principal components of the g tensor are normally used to identify the radical and are sensitive to the local electric field due to the surrounding charges. For 02near a Ti4+ion g,, = 2.02-2.03 (along the 0-0 axis); the other two g values are ca. 2.009 and 2.003.3 Recently we have studied perfluorinated Nafion ionomers neutralized by Ti3+ (as TiC13), and swollen by water and by methanol. Isolated Ti3+ cations ligated to oxygen ligands, Ti3+-Ti3+dimers, and possibly higher aggregates, have been detected in a wide range of cation concentrations and solvent contents.8-10 *TOwhom correspondence should be addressed. ‘Present address: Sola/Barnes Hind, 810 Kifer Road, Sunnyvale, CA 94086-5200.
0022-3654/89/2093-7526$01.50/0
This study was motivated by our interest in detecting differences in the local environment and especially reactivity for the isolated and aggregated cations in ionomers. We report results on the formation of 02-by electron transfer from Ti3+ in Nafion neutralized by Ti3+and by Ti3+/A13+mixtures of various compositions. Experimental Section The Nafion 1 17 sulfonated membranes, indicated in the formula below, had an equivalent weight of 1100 g/mol of S03H and a thickness of 0.13 mm, and were obtained from DuPont. -CF$F,CF-
I
[O-CF2CFln-OCF~-SO3H
I
CF3 Nafion, with n = 1
The membranes were pretreated as described in our previous publications8-10and dried to constant weight in vacuo ( lo4 Torr) for 24 h at ambient temperature and for 2-12 h at 373 K . (1) Che, N.; Tench, A. J. In Aduances in Catalysis; Academic Press: New York. ~ ,~ -1983: - . .Vol. -32. (2) Fridovich, I. In Superoxide Dismutase; Oberley, L. W., Ed.; CRC Press: Bwa Raton, FI, Vol. 1. (3) Shiotani, N.; Moro, G.; Freed, J. H. J . Chem. Phys. 1981, 74, 2616. (4) Anpo, M.; Shima, T.; Fujii, T.; Suzuki, S.; Che, M. Chem. Lett. 1987, 1997. (5) Kara, Y.; Matsukaze, Y. J . Chem. Phys. 1986, 90, 5752. (6) Gesser, H. D.; Kruczynski, L. .I. Phys. Chem. 1984, 88, 2751. (7) Ragai, J. Nature 1987, 325, 703. (8) Schlick, S.; Sjoqvist, L.; Lund, A. Macromolecules 1988, 21, 535. (9) Schlick, S.; Alonso-Amigo, M. G. J . Chem. SOC.,Faraday Trans. 1 1987, 83, 3575. (10) Alonso-Amigo, M. G.; Schlick, S. Polym. Prepr. (Am. Chem. SOC. Polym. Diu. Prepr.) 1987, 28, 363. ~
--.
0 1989 American Chemical Society
Of Formation in Nafion
77
The Journal of Physical Chemistry, Vol. 93, No. 21, 1989 I527
Ki
X Ti3+ 12 HOUHS
20 ,
-I
-3250
I HOUR
3350.
3450.
3550.
3650
Figure 1. X-band ESR spectra at 77 K of dry Nafion fully neutralized by Ti3+ and exposed to oxygen, as a function of the time of contact with the oxygen.
TiCI3 (Aldrich) and AICI3 as the hexahydrate (Baker) were used as received to prepare water solutions. Nafion equilibrated with Ti3+or with the Ti3+/A13+mixture was prepared by using 0.03 M TiC13 and 0.1 M AlC13 solutions in water under dry nitrogen. Quantitative analysis of the membranes (Galbraith Laboratories) indicated a negligible CI- content (1 1 ppm). This is due to the low permeability of the membranes to anionic species. To obtain the Of radical, the neutralized Nafion samples were transferred to 4 mm 0.d. quartz ESR sample tubes, dried under vacuum ( lo4 Torr) at ambient temperature for about 24 h, and exposed to oxygen in the vacuum line. ESR spectra were taken periodically, to follow the formation of 0,with time. ESR spectra at X-band were measured with a Bruker 200D SRC spectrometer operating at 9.7 GHz (empty cavity at ambient temperature) and 100 KHz modulation, interfaced with a data system based on an IBM PC and the software EPRDAS (Mega Systems solutions, Rochester, NY). Spectra at 77 K were taken in a liquid nitrogen Dewar inserted in the ESR cavity. Above this temperature the Bruker variable-temperature unit ER 41 11 VT was used. The absolute value of the magnetic field was measured with Micro-Now gaussmeter Model 515B, and with 2,2-diphenyl- 1-picrylhydrazyl (DPPH, g = 2.0036) and Cr3+ in MgO (g = 1.9796) standards.
Results X-band ESR spectra of Nafion fully equilibrated with Ti3+, as a function of the time of exposure to oxygen, are shown in Figure 1. The broad high-field signal is due to Ti3+ and has been characterized in a previous study.9 The low-field signal is typical of a radical with rhombic symmetry and is assigned to 02-.The g-tensor components are as follows: g,, = 2.0029, gyy= 2.0090, and g,, = 2.0193, f0.0005. These values are within the range deduced for 0, in systems containing Ti4+ions.3 An axial g tensor was reported for 0,in Nafion neutralized by methanol solutions of Ti3+, dried, and exposed to oxygen." The signal from Of increases, while that from Ti3+decreases, with time. These results can be explained by assuming the electron-transfer process Ti3+ O2 Ti4+ 02-.Ti4+ is diamagnetic and therefore is not detected in these experiments. The signal from Of is stable for at least 6 days at ambient temperature. The signal from Ti3+is still detected at high gain, when the signal from 02-is constant, indicating that not all of the Ti3+ions are involved in the electron-transfer process. In the absence of oxygen the signal from Ti3+is stable. Qualitatively, it appears that the amount of 0; formed is less than the decrease in the Ti3+ signal,
+
-
+
( 1 1) Fan, F. R. F.; Liu, H. Y.; Bard, A. J. J. Phys. Chem. 1985,89,4418.
3250.
3350.
3450.
3550.
3650.
Figure 2. X-band ESR spectra at 77 K of dry Nafion fully neutralized by Ti3+/A13+mixtures, exposed for 1 h to oxygen, as a function of mole percent Ti3+ in the cation mixture. E-,
I N NRFICN
4.00
3.50
L
1.50
A
./I
4 /
0
IO
23
30
40
50
/
60
j; i
70
I 80
PERCENT 4i2+ Figure 3. Relative intensity of the ESR signal from OF, expressed as the amplitude ratio h l / h l in Nafion samples neutralized by Ti3+, dried, and exposed for 3 h to oxygen.
indicating other reactions leading to the disappearance of the Ti3+ ions. The line shapes of the 02-signal do not change significantly in the temperature range 77-300 K; only a small shift of the low-field line (g,,) to higher fields and no measurable shifts in the positions of the other signals from 02-have been detected. These reversible changes are most likely due to motional effects. Above 300 K the signal from Of becomes weaker and above 330 K an irreversible change leads to the appearance of an isotropic signal centered on g = 2.0020; this signal might be due to an alkyl radical, from the reaction of the superoxide radical with the polymer substrate. In Figure 2 we present the ESR spectra obtained for dry Nafion membranes fully equilibrated with Ti3+/A13+mixtures. All spectra are taken at 77 K, after 1-h exposure to oxygen. The intensity of the signal from 0, is significantly larger, compared with that from Ti3+, in samples containing a higher percentage of A13+ in the cation mixture. This effect is shown in Figure 3, where the amplitude ratio of the two signals, as defined in the inset, is plotted as a function of mole percent AI3+in the cation mixture. The enhancement of the 0, intensity is especially evident for samples containing more that 50 mol 7% A13+ in the cation mixture. If Nafion samples containing Of are evacuated to lo-" Torr and heated briefly to 350 K, the spectrum shown in Figure 4 is observed; this spectrum is assigned to molecular oxygen freely rotating in the membranes.',
7528 The Journal of Physical Chemistry, Vol. 93, No. 21, 1989
Alonso-Amigo and Schlick
400 G
,F..-+;-t--t--c--+---*-~+.-+.35m. 460. 5400. miu. Y:’D0.
~i
Figure 4. ESR spectrum of freely rotating molecular oxygen, trapped in Nafion membranes treated as detailed in the text. The signal from Ti3+ at ca. 3500 G is also observed.
Discussion One of the factors affecting the rate of 02-formation is the oxygen permeability of the membrane. For a given cation mixture, the rate of 02-formation is lower for drier membranes. This is in accord with data in the literature that suggest the disappearance of the connecting channels between the polar domain in very dry Nafion and the reduction of the oxygen diffusion constant to a value close to that in poly(tetrafl~oroethylene).~~In “wet” membranes 02-is not formed, possibly because of its reduced stability in protic media.14 The experiments with the cation mixture indicate that the presence of A13+ ions is another important factor affecting the rate of 02-formation. This dependence, together with the detection of Ti3+-Ti3+dimers and clusters in dry Nafion neutralized by Ti3+! seems to indicate that the reactivity of Ti3+in clusters in the electron-transfer reaction is reduced. Similar conclusions about the reduced reactivity of clustered Ti3+have been obtained in a study of olefin polymerization by Ziegler-Natta catalysts on magnesium-based supports.I5J6 An enhanced photocatalytic activity of TiO, species for the hydrogenation of propene with water and for isomerization of cis-2-butene has been observed in the presence of A1203.17 This effect has been correlated with a decrease in the particle size of the catalyst with increasing A1203content. The photolysis of the catalyst involves most likely Ti3+, which is the reactive species in this study. In this respect the two studies are related. (12) Tinkham, M.; Strandberg, M. W. P. Phys. Rev. 1955, 97, 937. (13) Chryssikos, G.;Mattera, V. D. Jr.; Tsatsas, A. T.; Risen, W. M. Jr. J . Caral. 1985. 93. 430. (14) Narayana,’P. A.; Suryanarayana, D.; Kevan, L. J . Am. Chem. SOC. 1982, 104, 3552. (15) Brant, P.; Speca, A. N. Macromolecules 1987, 20, 2740. (16) Chien, J. C. W.; Hu,Y. J . Polym. Sci. Polym. Chem. Ed. 1989, 27, R91 - _ ..
(17) A n p . M. Reo. Chem. Intermed. 1989, 1 1 , 67.
Figure 5. X-band ESR spectra at 100 K of Nafion containing 33% Ti3+, for water-saturated dry blotted and dried membranes (2 h at 298 K in a vacuum of lo-“ Torr). The half-field transitions corresponding to Am, = 2 were obtained by increasing the receiver gain as indicated.
In principle the effect of the added A13+ ions on the formation of Ti3+-Ti3+dimers and higher aggregates should be observed as a decrease in the relative concentration of the half-field Am, = 2 transition, compared with the intensity in the g r 2 region. In practice, the dimer signal is reasonably strong only in membranes that have been fully equilibrated with Ti3+; only a small increase in the intensity of the signal is observed on removing the solvent? The dimer signal for membranes equilibrated with 33% Ti3+(no AI3+)and the effect of drying are shown in Figure 5. The dimer signal is too weak to be quantitatively used for the study of the A13+effect. The Ti3+signal in Figure 5 in dried membranes is very similar to that seen in Figure 2 for membranes containing a similar amount of Ti3+in the Ti3+/A13+mixture, 33% and 35%, respectively. It seems reasonable to assume that the line shape reflects the specific ligation of Ti3+for a given cation content and not the presence of A13+ ions. The results obtained in this study are relevant to the isotactic polymerization processes occurring on Ziegler-Natta catalysts. The advantage of the system presented here for the study of the cocation effect is its simplicity: In Nafion neutralized by Ti3+ we know that the ligands are oxygens (from the solvent and from the sulfonic groups of the network). In addition, we work under conditions that we can analyze the amount of the active species, in this case Ti3+. It is also possible that the Nafion membranes have the ability to control the size of the cation cluster that is formed; this aspect is currently under investigation in our lab with Ti3+and other paramagnetic cations, in the presence of different cocations. Acknowledgment. This research was supported by the Polymer Program of the National Science Foundation, through grant DMR-8718947 (ROW). The X-band ESR spectrometer was purchased through N S F equipment grant DMR-85013 12. Registry No. 02,7782-44-7; Ti3+, 22541-75-9; AI3+,22537-23-1; OF, 11062-77-4; Nafion, 39464-59-0.
